1. Field of the Invention
This invention relates to drilling generally horizontal boreholes in subterranean formations such as coal seams, and more particularly to a device which facilitates pumping a survey probe into a borehole and removing it therefrom.
Coal seams often contain methane gas in amounts which affect the safety of a mining operation. One method of alleviating the methane gas problem is to drill one or more boreholes into the seam in advance of mining, and to produce gas through the borehole until the methane gas level is low enough to permit safe mining. It is essential in such an operation to maintain the borehole within the coal seam, and techniques have been devised to provide this capability. One such technique utilized a survey probe which contains instrumentation capable of providing information regarding the borehole position. Such probes are periodically pumped down the interior of the drill string to a position adjacent the drill bit, readings are taken, and the survey probe is then removed from the drill string. If necessary, adjustments are made in the drilling operation, based on the information obtained, to maintain the borehole within the coal seam.
2. The Prior Art
The operation in which the present invention is applicable is described in detail in U.S. Pat. No. 4,295,536 which describes a method for periodically running an instrumented survey probe through a conduit to obtain information about a borehole being drilled. The present invention is intended for use in a similar operation.
Wireline retrievable tools having check valves for enabling fluid in a conduit to bypass the tool are known, and one such device is described in U.S. Pat. No. 3,965,978. The device described therein differs from the device according to the present invention in that it does not have a sealing member between the tool and the conduit, and the wireline connected thereto does not extend through the valve portion to a probe downstream thereof.
The present invention is particularly suited for use in connection with a pumpable instrument survey probe in connection with drilling generally horizontal boreholes as described in the aforementioned U.S. Pat. No. 4,295,536.